580 W. M. DAVIS — DATES OF TOrOGEAPHIC FORMS. 



the late or post-Tertiary cycle, here again the name taken for the geological 

 date of the uplift by which the cycle was opened being only roughly suggest- 

 ive of the time of its beginning. The work of this young cycle is so evident 

 that little time need be given here to its consideration. All the rivers are 

 now at work trenching their lowlands to depths of one, two, or three hun- 

 dred feet. In the course of the Susquehanna through the Appalachian low- 

 lands, this is admirably shown at many points. The Delaware, Lehigl^ 

 Schuylkill, and Susquehanna all cross the great valley in trenches below its 

 general surface, leaving it as an upland of low altitude — one, two, or three 

 hundred feet — to be again surmounted by remnants of the older Cretaceous 

 upland, higher still. The elevation of the Tertiary lowlands near the coast 

 may be taken as a measure of their uplift, but in the interior their present 

 altitude is certainly in part due to their imperfect reduction to baselevel in 

 the Tertiary cycle : for example, the Berkshire valley in Massachusetts and 

 the upper part of the Shenandoah valley in Virginia are now about 1,000 

 feet above sea-level, and part of this height may be ascribed to the 

 altitude of the Tertiary lowdand. The Delaware, Schuylkill, and Susque- 

 hanna traverse the Triassic lowland in the same way. When the traveler 

 crosses the Delaware from New Jersey to Pennsylvania by the Bound 

 Brook railroad (the so-called "New Line" between Philadelphia and New 

 York), the trench cut by the river in the even surface produced on the Tri- 

 assic area by Tertiary baseleveling is wonderfully v^ell shown. At the time 

 of the Tertiary baseleveling the Delaware appears to have been flood-plained 

 and its side streams turned down the valley, as were those of the Susque- 

 hanna in the latter part of the Cretaceous cycle. The Doylestown, Pennsyl- 

 vania, sheet of the Geological Survey topographic maps exhibits the evidence 

 of this in great perfection, as well as some interesting results following from 

 it in the readjustment of the superimposed streams after the post-Tertiary 

 elevation. The further search for downward deflected and partly readjusted 

 tributaries offers an attractive problem for students on the middle courses of 

 our larger rivers. 



The well-defined narrow and young trenches cut by the upper Ohio and 

 its branches at the bottom of relatively wide open valleys, described by 

 Chamberlin in his introduction to Bulletin 58 of the Geological Survey, 

 seem to correspond with those of the post-Tertiary cycle nearer the sea- 

 coast. 



During the post-Tertiary cycle the smaller streams have not sunk their 

 valleys much below the general surface of the lowlands, except in the neigh- 

 borhood of the larger rivers. In the belts of harder rocks the post-Tertiary 

 cycle has hardly done more than to freshen up the slopes ; thus, where the 



